1. The Field of the Invention
The present invention relates to a method of evaluating the surface characteristics of opaque materials. More particularly, the present invention is directed to a method of evaluating qualitative and quantitative surface characteristics of a opaque film by measuring the intensity of reflectance therefrom across a spectrum of electromagnetic wavelengths, where measurements are taken with a reflectance detector at a plurality of reflectance angles.
2. The Relevant Technology
It has become desirable to measure or otherwise analyze surface characteristics such as the microscopic surface roughness of certain opaque materials during the fabrication of the opaque materials. An example of one material for which it is highly beneficial to measure surface characteristics is polycrystalline silicon (polysilicon). Polysilicon is used in the semiconductor manufacturing industry as a conductive layer and has significant advantages over other conductive materials in that it can withstand the high temperatures that are often required in subsequent manufacturing steps, and because it has the same electron affinity as single crystal silicon. Polysilicon is a solid material which is comprised of pure silicon crystallites or "grains" separated by grain boundaries. Consequently, the morphology of polysilicon is generally characterizable by the size of the individual crystals and the width of the grain boundaries that separate the crystals.
Polysilicon is typically formed as a film on silicon wafers in the process of manufacturing integrated circuits. Careful control of reactant gas flow, temperature, and pressure is required for consistent polysilicon film production. Within even these narrow parameters, however, the surface qualities of the polysilicon film can vary dramatically.
A recent development in polysilicon applications involves growing intentionally rough films for use as capacitor plates in integrated circuits. The roughness of the surface of these films serves as a means to increase the effective surface area of the capacitor plate while occupying a minimum of wafer space. When the polysilicon is grown with large grains, in the order of 60 mm or more, it is considered hemispherical grain polycrystalline silicon (HSG polysilicon). HSG polysilicon is preferred for other semiconductor manufacturing processes as well, and in each case must be deposited under the proper conditions to maximize its surface area.
HSG polysilicon is typically formed in one of two manners. In the first manner of polysilicon formation, the HSG polysilicon is formed by chemical vapor deposition with an appropriate chemistry, typically comprising silane in an appropriate chamber under certain predetermined process conditions. The second technique comprises depositing a planar smooth film of highly amorphous polysilicon which is appropriately doped with a seeding dopant such as phosphine, arsine or disilane. These dopants are then used as nucleation sites for forming grains within the crystalline structure of the polysilicon. The grains are formed during an anneal step during which the film surface rearranges itself into grains, providing a rough surface area.
Integrated circuits are currently grown in large batches, and inadvertent change of even one parameter of the HSG production process could reduce the surface area of the film to such a degree as to cause a failure condition. As an example, when an HSG polysilicon film being used to form capacitor plates is deposited under less than optimal conditions, the surface area of the capacitor plate may be insufficient, resulting in capacitors that fail to hold a charge sufficiently. When the capacitors are used to form a DRAM memory cell, for instance, the DRAM memory cell will as a result fail to meet refresh rates. A defect condition results which can reduce fabrication processing yield significantly.
Consequently, the precise control of polysilicon deposition is desirable to the preferable practice of semiconductor manufacturing processes. In order to maintain the necessary control over the manufacturing process, a method of evaluating the surface roughness of the HSG polysilicon is needed, both for process development and in-process monitoring. Furthermore, the method needs to be flexible, in order to meet the very different demands of both process development and in-process monitoring.
Process development requires a method that is accurate and dependable, and capable of providing detailed information as to specific surface characteristics, including at least surface roughness, grain size, and surface area. An in-process monitoring method need not necessarily provide highly detailed information, but should at least be able to determine when a variation in surface characteristics takes place. It should also be suitable to be conducted in-situ, and should not lower throughput.
Aluminum is a further example of a material for which a method of evaluating the surface characteristics is needed. In practice it has proven difficult to maintain the deposition parameters for aluminum at appropriate levels in order to result in a smooth surface of the deposited aluminum. In order to verify the smoothness of deposited aluminum and determine exactly how the parameters must be adjusted to maintain a smooth surface, a method for the in-process determination of the surface roughness of the aluminum is desirable.
The prior art has employed a number of methods for evaluating the surface characteristics of opaque materials. None of the prior art methods, however, has proven fully satisfactory for both product development and in-process evaluation of the surface characteristics of materials such as HSG polysilicon and aluminum.
One method previously used for analyzing surface characteristics of substrate materials is scanning electron microscopy (SEM). Using the SEM method, the sample is bombarded with electrons, and the electrons are then measured for surface characterization information as they return to the device. One limitation of this method is that the electrons do not penetrate deeply into the sample. Thus, SEM has proven helpful for analyzing lateral dimensions of surface roughness, but is very limited in analyzing vertical dimensions. Furthermore, SEM is generally destructive of the sample, has slow feedback times, and cannot be used in-process, during fabrication. It is also difficult to determine qualitative data about the sample using SEM. Furthermore, SEM cannot be used to determine precise grain size and surface area of highly granular samples. SEM analysis would be highly dependent upon the operator's judgment for such a characterization.
Another method previously used is atomic force microscopy (AFM). AFM utilizes a very small stylus, similar to a record needle. The stylus is scanned across the sample, back and forth over a small area, while a laser is reflected off of a platform located on the stylus. The deflection of the stylus is then measured by the variations in the returned laser light. The laser detector detects the reflected laser light, which is a direct result of the vertical movement of the stylus. This method provides very high resolution, even down to atomic resolution for certain samples, but has proven incapable of producing repeatable results. Furthermore, AFM is a very technical and demanding process, requiring highly-trained operators. AFM is also highly susceptible to environmental noise and surrounding vibrations. Additionally, the AFM contact method is somewhat destructive. While it does not destroy a whole wafer, it does destroy at least the part that is being tested. Thus, AFM has proven impractical for implementation on an in-process basis.
A further method previously used in the art is tunneling electron microscopy (TEM). Using TEM, a sample is prepared on a very thin slice of silicon, then electrons are bombarded through the sample. The density of the electrons are measured on the other side of the sample with a detector. The pattern the electrons make on the detector as a result of the material that is being passed through is used to determine the size of the grains and their locations. This is also a very sensitive technique which can measure down to a very small feature size and can give calibrated results. Once again, however, the process is very time consuming, and involves a very high skill level of the operator. TEM is also a destructive method that cannot be conducted in-process.
A further method of surface evaluation is described in a paper entitled "Rapid Characterization of Polysilicon Films by Means of a UV Reflectometer" by G. Harbeke, E. Meir, J. R. Sandercock, M. Tjetjel, M. T. Duffy, and R. A. Soitis, RCA Review, Vol. 44, March 1983. Therein, a method of characterization of polysilicon films by means of a UV reflectometer is taught. The UV reflectometer is used to measure the reflectance of polysilicon on semiconductor wafers at one of two fixed wavelengths in the ultraviolet spectral region. Particularly, it measures the wavelengths at either 280 nm or at 400 nm, depending upon the application. The wavelength of 280 nanometers is used for in-process quality control of dust and defect detection. The wavelength of 400 nanometers, which has been found to probe to greater depths, is disclosed as being used to probe for bulk structural perfection in polycrystalline films.
The reflectometer uses a deuterium lamp, which provides a continuum light source. A chopper is used to alternately reflect light to a sensor from the sample and from blades on the chopper. The detector uses a silicon photovoltaic device with an enhanced UV response to detect the fraction of light reflected. The electrical signals from the detector are used to form a second signal which is normalized to the difference of the reflectance from the chopper minus the reflectance from the sample. The result is then plotted by reflectance and corresponds generally to surface smoothness and defect conditions.
This method has proven satisfactory for measuring surface defects and bulk characteristics of highly polished polysilicon films. Once again, however, sufficient information is not provided to characterize HSG polysilicon films. Such data from one of the wavelengths at 280 nm and 400 nm becomes somewhat inconsistent when the silicon passes a certain roughness, and provides only a moderate degree of information about the surface area of rougher films. Furthermore, no method of quantifying the raw reflectance data into useful information about surface characteristics is provided.
In order to better understand why the above method is inadequate for characterizing HSG polysilicon films, the graphical depictions of FIGS. 1 through 4 are provided. Therein are shown four different samples of HSG polysilicon. The sample of FIG. 1 has a relatively small grain size, uniform grains, and large boundary spaces between the grains. The sample of FIG. 2 is of a larger grain size and still of relative uniformity. In FIG. 3, an even larger grain size is depicted, with relatively uniform grains. FIG. 4 depicts a yet larger grain size, with somewhat irregularly shaped grains. The HSG polysilicon films graphically depicted in FIGS. 1 through 4 may be used, by way of example, as capacitor plates in integrated circuits.
The single wavelength method described above will detect a difference in reflectance between each of these samples, but has been found incapable of transforming that difference into a quantification of surface characteristics or of predicting surface area with a high degree of precision. This is due in part to the lack of information provided by each single wavelength about surface characteristics such as grain size, grain shape, grain density, and grain boundary size, all of which affect the total surface area and will effect the measured reflectance differently.
Thus, from the above discussion it can be seen that the need exists in the art for a method of evaluating the surface characteristics of opaque materials such as polysilicon films during process development. Furthermore, there is a need for a method of evaluating the surface characteristics of such materials which can be conducted in-process without destroying the sample, which can be conducted rapidly, and which can be conducted by operators that are not extensively trained. Such a method is particularly needed which is highly accurate in quantitatively determining particular surface characteristics such as surface area under varying parameters, and which can be used for opaque films of a high surface roughness.